Measurements and seasonal changes in weight of guillemotsUria aalgeat a breeding colony

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Abstract
We caught, weighed and measured Guillemots at a colony before and during the breeding season, and when they returned to the colony in October following the main moult of the year. Their sex and breeding status were assessed by detailed observations. Males were heaviest in the prelaying period and lightest when they had young. Lost weight was regained by the autumn. Females showed a similar pattern but the changes were less marked. Adults showed a significant decline in weight with increasing age of the chick. Nonbreeding birds had similar wing lengths to adults but were lighter in weight. We could detect no effect of weight in the prebreeding period on whether or not birds bred, when the egg was laid or breeding success.